Printed circuit board for electrical and optical signals and method for producing the same

ABSTRACT

The invention relates to a printed circuit board ( 30 ) which has at least one electrical conduction level (EL) for relaying electrical signals and/or currents and at least one optical conduction level (OL) for relaying optical signals. Said conduction levels (EL, OL) are placed on top of each other in a stack inside the printed circuit board ( 30 ) and are interconnected. The aim of the invention is to provide a particularly flexible and simple construction and a simplified production method. To this end, the optical conduction level (OL) comprises at least one thin glass layer ( 11 ) as a conductor element.

TECHNICAL BACKGROUND

The present invention relates to the field of circuit board technology.It concerns a circuit board having at least one electrical conductionlevel for relaying electric signals and/or currents and at least oneoptical conduction level for relaying optical signals, said conductionlevels being interconnected and arranged one above the other in a stackwithin the printed circuit board.

Such a circuit board is disclosed, for example, in the publication U.S.Pat. No. 5,408,568 A.

This invention also relates to a method of producing such a circuitboard.

STATE OF THE ART

In the long run, the demand for higher and higher clock rates and fasterand faster signal transmission cannot be met with an adequate quality byusing copper lines. Through the use of optical transmission pathways(optical fibers) it is possible to transmit signals at extremely hightransmission rates within a backplane and also on system boards. A highinterference immunity with respect to electromagnetic interference is avery fortunate side effect which is important especially on an electricbackplane. Such a backplane is responsible, for example, for the dataexchange between the individual processor cards of a multiprocessor highperformance computer.

Various proposals have already been made for integrating optical datatransmission pathways into multilayer circuitboards. For example, U.S.Pat. No. 5,230,030 A describes an optical interface for coupling to anelectronic circuit in the form of multichip modules. The individual ICsare mounted on a multilayer circuit board consisting of layers thatconduct electricity, arranged in alternation with insulating layers inthe form of a stack. Channels are created in the insulating layers,which consist of an optically transparent material having a lowrefractive index, and are then filled with another transparent plastichaving a higher refractive index. The fillings then form opticalconductors which are connected to the ICs on the one hand and on theother hand are led to the edge of the stack, where they can be connectedexternally by means of an appropriate plug. This method of integratingoptical conductors into a circuit board is not only complicated becausethe circuit board must be built up and structured layer for layer insuccession, but also the quality of the optical conductors produced inthis way leaves much to be desired because it is very difficult toachieve a uniform and homogeneous conductor structure when filling thechannels with the optically active material.

Another proposal which is disclosed in U.S. Pat. No. 5,408,568 A citedabove integrates a whole-area optical layer as an intermediate layerinto a multilayer circuit board. The optical layer is coupled to theoutside by means of an optical fiber with a blunt connection on one end.Chips placed on the top side of the circuit board are connected throughholes beneath the chips, extending to the optical layer. The opticallayer acts as a uniform optical databus over which all chips canexchange data with each other or with the outside world. No statementsare made in that publication regarding the thickness or material of thisoptical layer. FIG. 1 of that publication illustrates the optical layerwith the same thickness as the printed circuitboards between which it isarranged. Therefore, this type of design could not be suitable forcircuitboards having multiple optical levels and defined opticalconnections between selected chips.

EXPLANATION OF THE INVENTION

Therefore, the object of this invention is to create a circuit board forelectrical and optical signals which is characterized by a high qualityof the optical connection, a flexibly adaptable and easily varied designand simple integration into known manufacturing methods for multilayercircuitboards.

This object is achieved with a circuit board of the type defined in thepreamble by the fact that the optical conduction level as a conductingelement comprises at least one thin glass layer. A thin glass layer isunderstood to refer to a sheet-like layer of glass with a smallthickness (approximately 1 mm thick or less) but at the same time a highoptical quality (planarity of the surfaces) such as that used for LCDdisplays, solar cells or as a cover for a CCD circuit. Due to the use ofsuch thin glass layers, it is possible to provide within the circuitboard one or more space-saving optical conduction levels of a hightransmission quality which can also be structured easily as needed toimplement a localized optical connection within the circuit board,

A first preferred embodiment of the circuit board according to thisinvention is characterized in that the optical conduction level isformed by an optical sandwich which comprises, in addition to theminimum of one thin glass layer, at least one carrier plate which isconnected to the minimum of one thin glass layer over the surface. Dueto the combination of the thin glass layer with a carrier plate, it ispossible to prefabricate the resulting optical sandwich separately fromthe fabrication of the actual circuit board and to adapt it in aflexible manner to the prevailing needs of the circuit (e.g. bystructuring). The prefabricated optical sandwich can be introduced as anadditional conduction level or layer into a traditional manufacturingprocess for a multilayer circuit board without requiring any significantchanges in the process management.

It is possible here for the optical sandwich to comprise at least twocarrier plates with the minimum of one thin glass layer arranged betweenthem. The thin glass layer is then completely protected for furtherprocessing. However, it is also equally possible for the opticalsandwich to comprise at least two thin glass layers which are connectedto the minimum of one carrier plate over the area, with either theminimum of two thin glass layers being arranged on one side of theminimum of one carrier plate and joined together over the area or theminimum of two glass layers being arranged on opposite sides of theminimum of one carrier plate. In this way, two different opticalconduction levels that can be designed separately can be integrated intothe circuit board per optical sandwich. Of course, the number of carrierplates and thin glass layers per optical sandwich can also be increasedfurther within the scope of this invention, although this does makeproduction more complicated.

A second preferred embodiment of the circuit board according to thisinvention is characterized in that the carrier plates are made of anelectrically insulating material which is used as the basic material forthe production of electric circuitboards, preferably anAramid-reinforced resin. This guarantees that the optical sandwich canbe introduced especially well into the traditional manufacturing processfor multilayer circuitboards.

The thin glass layers preferably have a thickness of less than or equalto 1.1 mm and are made of a borosilicate glass. Such a thin glass whichis available under the brand names AF 45 and D 263 from the Germancompany DESAG for use in LCD displays or solar cells, for example, andwhich is available in thicknesses between 30 μm and 1.1 mm is especiallysuitable as the optical conduction layer because of its high opticalquality.

Essentially the thin glass layer may remain unstructured, then forming asingle, continuous, cohesive optical layer. Another preferred embodimentof the circuit board according to this invention, however, ischaracterized in that at least individual thin glass layers arestructured in such a way as to form individual optical conductors withinthe layer, separated from one another by interspaces. In this way, avariety of independent optical conductors can be produced in one leveland can assume different transmission functions without causing anymutual interference.

The optical properties of the individual optical conductors can beoptimized either by covering the exposed surfaces of the individualoptical conductors with a reflective layer or by filling the interspacesbetween the optical conductors with a filling material which has a lowerrefractive index than the refractive index of the glass of the thinglass layer in particular.

Another preferred embodiment of the circuit board according to thisinvention is characterized in that coupling openings are provided foroptical coupling of optically active elements arranged on the top and/orbottom sides of the circuit board, so that the concealed thin glasslayer or optical conductors inside an optical conduction level is/areaccessible from the outside.

The method according to this invention for producing a circuit board ischaracterized in that in a first step, at least one thin glass layer isjoined to at least one carrier plate over the entire area to form anoptical sandwich, and in a second step the optical sandwich is connectedto the circuit board as an optical conduction level having one or moreelectrical conduction levels in a stack arrangement, with the thin glasslayer and the carrier plate preferably being joined together by pressingand/or gluing.

Additional embodiments are derived from the dependent claims.

BRIEF EXPLANATION OF THE FIGURES

This invention will now be explained in greater detail below on thebasis of embodiments in conjunction with the drawings, which show:

FIGS. 1A-D various stages in the production of an “optical sandwich”with a structured thin glass layer according to a preferred embodimentof this invention, shown in a perspective, partially cut-away view;

FIG. 2 an alternative embodiment to FIG. 1D with a thin glass layerbetween two carrier plates;

FIG. 3 an alternative embodiment to FIG. 1D with two thin glass layersarranged one above the other on one side of the carrier plates;

FIG. 4 an alternative embodiment to FIG. 1D with two thin glass layerson opposite sides of the carrier plate;

FIG. 5 an enlarged sectional diagram of the optical sandwich accordingto FIG. 1D with the interspaces of the structured thin glass layerfilled by an optically adapted filling material;

FIG. 6 an alternative embodiment to FIG. 5B with a cover over thestructured thin glass layer formed by a reflective layer; and

FIG. 7 an embodiment of a circuit board according to this invention withthree optical conduction levels (sandwiches) according to FIG. 2,separated from one another by two electrical conduction levels arrangedbetween them.

METHODS OF EMBODYING THIS INVENTION

In production of the circuit board according to this invention,individual so-called “optical sandwiches” are produced first, to formthe optical conduction level in a future circuit board. The opticalsandwich is produced in several steps which are shown as an example inFIGS. 1A-1D, starting with a carrier plate 10 (FIG. 1A), which hasplaner surfaces on the top and bottom sides and is made of anelectrically insulating material such as that used for the production ofelectrical circuitboards. This ensures that the finished opticalsandwich can be integrated well into existing circuit board processesfrom the standpoint of its material properties. The material used forthis is preferably an Aramid-reinforced resin. Such carrier plates areavailable under the brand name Duramid-P-Cu 115ML from the Germancompany Isola, for example. However, any other insulation materialhaving isotropic properties and a coefficient of expansion like that ofglass can also be used. The thickness of carrier plate 10 is selected sothat carrier plate 10 imparts a sufficient mechanical stability to theoptical sandwich, but on the other hand does not take up an unnecessaryamount of height in subsequent integration into the circuit board.

Carrier plate 10 is then joined to a thin glass layer 11 by pressing orgluing over the entire area according to FIG. 1B. The thin glass layer11 is preferably made of a borosilicate glass and has a thickness ofless than or equal to 1.1 mm (30 μm to 1.1 mm). Thin glass of this typeis available under the brand names AF 45 and D 263 from the Germancompany DESAG, for example. Thin glass AF 45 is a modified borosilicateglass with a high BaO and Al₂O₃ content and is characterized by a lowthermal expansion coefficient and a high light transmission value. Dueto low tolerances and flame-polished surfaces, this type of glass isespecially suitable for large-area optical applications such as LCDdisplays, covers for CCD elements, solar cells or the like. Thin glass D263 is a borosilicate glass with corresponding optical properties. Bothof these thin glasses are available in a thickness in the range between30 μm to 1.1 mm.

If the optical conduction level is provided only as a common databus inthe subsequent circuit board, then an optical sandwich can be integrateddirectly into the circuit board with an unstructured thin glass layeraccording to FIG. 1B (see FIG. 7). However, if individual opticalconduction connections are needed between different points in the plate,the thin glass layer is then structured according to FIG. 1C aftercreation of the optical sandwich by completely removing the thin glasslayer in certain areas to form interspaces 12 between individual opticalconductors 13. The individual optical conductors 13 may have differentareas (as shown in FIG. 1C). They may run parallel to one another andmay have the same or different lengths, but they may also be bent orshaped in some other manner in as much as this is consistent with theirfunction as an optical conductor. Interspaces 12 can be created bydifferent techniques. A mechanical method of production by grinding ormilling is conceivable, but removal by means of a laser or by chemicalmethods is also conceivable.

After the thin glass layer 1 has been structured, the resultinginterspaces 13 are filled with a filling material 14 to complete theoptical sandwich 15 (FIG. 1B and FIG. 5). This filling operation has theadvantage that a mechanically stable planer surface is formed on the topside of thin glass layer 11. On the other hand, if filling material 14has a refractive index lower than the refractive index of the glass ofthin glass layer 11, then this filling material ensures total reflectionin optical conductors 13 and thus ensures good optical conductionproperties. However, the same good optical conduction properties canalso be achieved if the free surfaces of the structured thin glass layer11 or the optical conductor 13 are coated with a preferably metallicreflective layer 29 by vapor deposition or by galvanic or chemicaldeposition in optical sandwich 15.4, as illustrated in

FIG. 6. Again in this case, the remaining interspaces may be filledsubsequently with a filling material for mechanical reasons.

Instead of the optical sandwich 15 from FIG. 1B composed of two layers10 and 11, optical sandwiches comprising more than two layers may alsobe used. In the case of optical sandwich 15.1 from FIG. 2, the thinglass layer 11 is joined to carrier plates 10 and 16 on both sides. Thisfurther increases its mechanical stability. At the same time, opticalsandwich 15.1 has the circuit board material of carrier plates 10 and 16as the connecting surface on the top and bottom sides and therefore itcan be integrated especially well into the circuit board manufacturingprocess.

In the case of the optical sandwich 15.2 from FIG. 3, a secondstructured thin glass layer 17 is arranged above the first structuredthin glass layer 11 and forms a second optical conduction level and thusprovides additional optical connections within the circuit board withouttaking up much space. In the case of the second thin glass layer 17, theinterspaces are preferably also filled by a filling material 18.

Finally, in the case of the optical sandwich 15.3 from FIG. 4, astructured thin glass layer 11 and 17 is provided with interspacesfilled with filling material 14 and 18 on the opposite sides of carrierplate 10, thus providing a clear separation between thin glass layers 11and 17. It is self-evident that other combinations of thin glass layersand carrier plates are also conceivable within the scope of thisinvention.

The finished optical sandwiches 15 and 15.1 through 15.4 can then becombined in a stack with traditional electric circuitboards that aremetallized on both sides and connected to form a finished circuit boardfor optical and electrical signals. The mechanical stability of thesesandwiches permits problem-free integration into the production process.One such circuit board 30 shown as an example has three opticalconduction levels and two electrical conduction levels, as illustratedin a sectional view in FIG. 7. The (three) optical conduction levels OLof the circuit board 30 are formed by three optical sandwiches 15.1according to FIG. 2. Two electrical conduction levels EL are arranged inalternation between the optical conduction levels OL, each consisting ofa dielectric layer 19 and 20 in a traditional manner, coated on bothsides with a metal layer 21, 22 and 23, 24 (e.g., copper lamination).All the layers are pressed or glued together. Both the thin glass layersof optical conduction levels OL and the metal layers of electricalconduction levels EL are structured according to the requirements ofcircuit board 30, where the structuring of electrical conduction levelsEL is accomplished in a known manner (e.g., by etching the metal layers21-24); structuring of metal layers 21-24 is not illustrated in FIG. 7for the sake of simplicity. Of course, through-contacts such as thoseknown and conventionally used in the technology of multilayercircuitboards may also be provided between electrical conduction levelsEL.

If optically active elements or chips 25, 27 are to be connected to eachother or to optical inputs or outputs, plug connections or the likethrough the optical conduction levels OL, then coupling openings 26, 28are introduced into the circuit board 30 for optical coupling of theelements 25, 27 arranged on the top and/or bottom sides of the circuitboard 30, these coupling openings providing access from the outside tothin glass layers 11 and optical conductors 13 which are concealed andlie in an optical conduction level OL. Accordingly, purely electronicchips which are arranged on the circuit board can be connected toelectric conduction levels EL by means of through-contacts (not shown inFIG. 7).

LIST OF REFERENCE NOTATION

10, 16 carrier plate

11, 17 thin glass layer

12 interspace

13 optical conductor

14, 18 filling material

15; 15.1-15.3 optical sandwich

19, 20 dielectric layer

21-24 metal layer (e.g., Cu)

25, 27 optically active element (optical chip)

26, 28 coupling opening

29 reflective layer

30 circuit board

EL electrical conduction level

OL optical conduction level

What is claimed is:
 1. A circuit board (30) having at least oneelectrical conduction level (EL) for relaying electrical signals and/orcurrents as well as at least one optical conduction level (OL) forrelaying optical signals, said conduction levels (EL, OL) beinginterconnected and arranged in a stack one above the other within thecircuit board, characterized in that the optical conduction level (OL)as a conducting element comprises at least one thin glass layer (11, 17)in the form of a prefabricated thin glass sheet made of a borosilicateglass and having a thickness of less than or equal to 1.1 mm.
 2. Thecircuit board according to claim 1, characterized in that the opticalconduction level (OL) is formed by an optical sandwich (15; 15.1 , . . ., 15.3) comprising, in addition to the minimum of one thin glass layer(11, 17), at least one carrier plate (10, 16) which is joined to theminimum of one thin glass layer (11, 17) over the area.
 3. The circuitboard according to claim 2, characterized in that the optical sandwich(15.1) comprises at least two carrier plate (10, 16) with a minimum ofone thin glass layer (11) arranged between them.
 4. The circuit boardaccording to claim 2, characterized in that the optical sandwich (15.2,15.3) comprises at least two thin glass layers (11, 17) which are joinedover the area to the minimum of one carrier plate (10).
 5. The circuitboard according to claim 4, characterized in that the minimum of twoglass layer (11, 17) are joined together over the area and are arrangedon one side of the minimum of one carrier plate (10).
 6. The circuitboard according to claim 4, characterized in that the minimum of twoglass layers (11, 17) are arranged on opposite sides of the minimum ofone carrier plate (10).
 7. The circuit board according to claim 2,characterized in that the carrier plate (10, 16) are each made of anelectrically insulating material which is used as the base material forthe production of electric circuit boards.
 8. The circuit boardaccording to claim 7, characterized in that said carrier plates (10, 16)are each made of an Aramide reinforced resin.
 9. The circuit boardaccording to claim 1, characterized in that the thin glass layers (11,17) and the carrier plates (10, 16) are glued or pressed together. 10.The circuit board according to claim 1, characterized in that at leastindividual layers of the thin glass layers (11, 17) are designed ascontinuous layers.
 11. The circuit board according to claim 1,characterized in that at least individual layers of the thin glasslayers (11, 17) are structured so as to form individual opticalconductors (13) within the layer, separated from one another byinterspaces (12).
 12. The circuit board according to claim 11,characterized in that the exposed surfaces of the individual opticalconductors (13) are covered with a reflective layer (29).
 13. Thecircuit board according to claim 11, characterized in that theinterspaces (12) between the optical conductors (13) are filled with afilling material (14, 18).
 14. The circuit board according to claim 1,characterized in that coupling openings (26, 28) are provided foroptical coupling of optically active elements (25, 27) arranged on thetop and or bottom sides of the circuit board (30), so that the concealedthin glass layers (11, 17) or optical conductors (13) located in anoptical conduction level (OL) are accessible from the outside throughthese coupling openings.
 15. A method of producing a circuit boardcomprising the steps of: joining at least one thin glass layer (11, 17)comprising a prefabricated thin glass sheet made of a borosilicate glassand having a thickness of less than or equal to 1.1 mm over the entirearea to at least one carrier plate (10, 16) to form an optical sandwich(15; 15.1, . . . , 15.3), and connecting the optical sandwich (15; 15.1,. . . , 15.3) to the circuit board (30) to form an optical conductionlevel (OL) having one or more electrical conduction levels (EL) in astack arrangement.
 16. The method according to claim 15, characterizedin that the thin glass layer (11, 17) and the carrier plate (10, 16) arejoined together by pressing or gluing.
 17. The method according to claim15, characterized in that the thin glass layer (11, 17) joined to thecarrier plate (10, 16) is structured between the first and second steps.18. The method according to claim 17, characterized in that the thinglass layer is removed in certain predetermined areas in order tostructure the thin glass layer (11, 17) to form individual opticalconductors (13) separated from one another by interspaces (12).
 19. Themethod according to claim 18, characterized in that the removal of thethin glass layer (11, 17) is accomplished by means of lasers or bymechanical or chemical methods.
 20. The method according to claim 17,characterized in that the free surface area of the structured thin glasslayer (11) is coated with a reflective layer (29), preferably made of ametal, by vapour deposition, galvanic or chemical deposition.
 21. Themethod according to claim 18, characterized in that the interspaces (12)in the structured thin glass layer (11, 17) are filled with a fillingmaterial (14, 18) having a refractive index lower than the refractiveindex of the glass of the thin glass layer (11, 17).